The natural teeth of a patient are often lost as a result of dental disease or trauma, making it desirable to replace a natural tooth with a prosthetic device. Treating with custom implants is ideal where the walls of the root are so thin that the problem tooth cannot be adequately treated with post and core, or in situations where the tooth has decayed to the point of involvement well below the crest of the alveolar bone. Dental arches that have short or long edentulous spaces, excellent bone quality, thickness and depth would be ideal situations where the treatment of choice is implants that are either preformed or custom made.
One type of prosthetic device is the dental endosteal implant that is intended to be surgically positioned into the bony structure (mandibular or maxillary alveolar bone), and be held there (a) by osseo-integration with the bony structure itself, (b) by ingrowth of fibrous tissue, (c) by the use of screws or self-retaining attachment mechanisms physically engaged to the bony structure, or (d) by any combination of the above. After healing the implant is fitted with a tooth-simulating prosthesis.
Since dental implants serve as the foundation for a dental prosthesis, implants must have sufficient mechanical strength and stability to withstand the forces and pressures generated during function (mastication etc.). The structural configuration of the implant and the manner in which it is installed in the jawbone are determinants in the ability of the implant to maintain its installed position over an extended period of time.
More specifically, one of the common causes of failure of traditional implants is excessive loading on a small section of the alveolar bone due to the inadequate distribution of loading forces. It has been found that screw implants, for example, exert six times the force of normal teeth on the alveolar bone generated by average vertical masticular loads. As a result, over an extended period of time the increased pressure applied to the alveolar bone and surrounding area may result in the implant coming loose from the alveolar bone.
There are two main types of conventional implants: press-fit and threaded. Both types are installed into a prepared recess made in the alveolar bone.
The press-fitted implant lacks a macro retention mechanism, making it vulnerable to movement. As a result, there is a need to provide apertures in the implant to permit ingrowth of bony tissue to ensure a rigid attachment. However, the implant as installed can still be vulnerable to movement particularly if fibrous tissue growth exceeds the amount of alveolar bone growth throughout the apertures of the implant.
For example, U.S. Pat. No. 3,979,828 issued Sep. 14, 1976 to Taylor discloses an implant that has a configuration shaped to encourage hard tissue and bone growth into and through portions of the implant for increasing the mechanical interlock between the implant and the existing alveolar bone. Taylor states that a healing period is required to ensure that the required mechanical interlock is achieved. However, even after this healing period the implant can still become loose during function, since the only mechanical anchor is provided by bone and fibrous tissue growth that may be weak. This is primarily due to the fact that press-fit systems rely on strong bone growth to anchor the implant, and if fibrous tissue formation dominates the attachment to the implant then the implant can easily be loosened during function.
A variation of the press-fit type of implant is disclosed in U.S. Pat. No. 4,854,873 issued Aug. 8, 1989 to Linden. The Linden implant is installed by press fitting the rod type implant into a cylindrical hole and then twisting to anchor the implant in the alveolar bone. Linden discloses a cylindrical implant having a series of projecting plates that are arranged in circumferentially spaced longitudinal rows along quadrants of the cylindrical surface. Since the projecting plates have no flutes, and are radially aligned, there is no biasing movement of the implant in a longitudinal direction during the twisting operation.
Due to the longitudinal alignment of the Linden projecting plates, the implant provides limited resistance to dislodging the implant in an unseated direction (i.e. vertically in an extraction direction). In addition, cylindrical implants poorly distribute compressive forces and generate shears forces that may fragment and break the bone surrounding the implant during function.
The threaded type implant provides, at least initially, more stability than a press-fit attachment, but the implant is still vulnerable to movement and high levels of shear forces can be established between the implant and the alveolar bone during function. Consequently, it is usually necessary to rely on growth of new bone tissue and connective tissue to further stabilize the installed position of the implant.
For example, U.S. Pat. No. 3,849,887 issued on Nov. 26, 1974 to Brainin discloses an implant having a serrated or dentated surface section and one or more expanded grooves to provide temporary mechanical interlock during the early stages of the implantation process. With threaded implants the bone that grows between the flutes of the thread on the implant can be sheared away and fragmented due to the high levels of shear stress generated at these points during function.
This is a significant area of weakness for a threaded implant, since the implant has a tendency to loosen in the socket and therefore must be removed and replaced by a larger diameter implant.
The use of a conically tapered implant to distribute compressive forces evenly to surrounding alveolar bone has been proposed in the previously mentioned U.S. Pat. No. 3,979,828 (Taylor). However, more favourable force distribution would be obtained if the implant taper closely matches the recess in the alveolar bone after a single rooted tooth has been extracted.
A process for making custom implants is disclosed in Canadian Patent Application No. 2,029,646 laid-open on May 10, 1991 to Propper. Among many other steps, Propper disclosed the step of preparing a model of at least the major portion of the root of the extracted tooth. Propper teaches that conventional moulding practices are used, such as the lost-wax/wax-up process, for making a replica of at least part of the tooth.
The problem with this type of conventional moulding practice is that the impression material (typically rubber, silicone etc.) is in direct contact with the alveolar bone and surrounding tissue during the stage when an impression is being obtained. This can cause the impression material to seep into the surrounding tissue and cause adverse reactions such as infections etc.
Consequently, there is a need for an implant that:
(a) provides immediate macro retention without using continuous self-tapping threads, and does not rely solely on bone growth to stabilize the implant; and PA1 (b) minimizes sheer forces and maximizes the even distribution of compressive forces to the alveolar bone generated during function (mastication etc.).
In addition, there is a need for a safe method for obtaining a direct bone impression from a recess in the alveolar bone in order to manufacture a custom implant.